Increasing Spring Temperatures Advance Post-Diapause Swarming and Prolong the Reproductive Period in the Bark Beetle Ips typographus

Diapause is a common strategy in insects to respond to reoccurring adverse events. The European spruce bark beetle Ips typographus is an important forest pest in Eurasia affecting spruce-dominated forests by eruptive outbreaks, which are likely to become more frequent and severe in a changing climate. It expresses a reproductive diapause to cope with harsh winter conditions. As diapause also affects the activity and voltinism of I. typographus, understanding its regulation by abiotic environmental factors can help to mitigate outbreak risks, for example, by coordinating management measures or adapting silvicultural strategies. While diapause induction in late summer and autumn has been shown to be mainly triggered by photoperiod and modified by temperature, information on environmental cues affecting post-diapause swarming in spring is scarce. In late winter/early spring 2021 and 2023, we conducted a laboratory experiment assessing spring swarming and reproductive potential of I. typographus after diapause termination, applying various temperature and photoperiodic conditions. We used mean temperatures between 13°C and 23°C with daily oscillations of ±5°C and daylengths between 9 h and 14 h (January–April). Post-diapause temperature sums had the strongest effect on spring swarming, modified by daily maximum temperatures, while photoperiod had only a small and ambiguous effect. In contrast, the reproductive potential of dispersing beetles remained almost unaffected by temperature sum, daily temperature and photoperiod. Our results show that spring swarming can potentially start as soon as cold temperatures have terminated diapause in mid-winter. Ongoing climate warming will consequently extend the reproductive season, likely increasing voltinism and population densities of I. typographus. Our findings can be implemented in phenology and risk assessment models to predict the temperature-dependent phenology and infestation risk in a future climate more accurately, to support bark beetle management (e.g., sanitation and salvage logging) and guide long-term silvicultural adaptations.